In the mechanical control of the travel of the elevator cars of an elevator system, motors, brakes and a control system to control them are needed. The driving force from the motor travels to the elevator car via the traction sheave and the elevator ropes. The hoisting machine of the elevator comprises one or more holding brakes, which ensure that the car stays in its position when movement of the elevator could endanger the safety of elevator passengers. If for some reason a malfunction occurs in the holding brakes, it could result in a hazardous situation for users of the elevator.
A defect can occur in the mechanical system controlling an elevator car, from which e.g. oil may be released into the equipment. The penetration of oil or other dirt onto the brake drum may reduce the friction co-efficient between the braking surface and the brake pad so much that the brake begins to slip. In other words it is possible that the elevator car moves when the brake shoe is engaged against the braking surface. The friction between the braking surface and the brake pad may also be reduced as a result of other factors, such as incorrect adjustment of the brakes. In addition, the brake pad may wear so that the brake is not able to keep the elevator in its desired position.
In prior-art technology the problems relating to holding brakes have been solved mainly with a regular maintenance program, with which it is ensured that the brakes are in proper condition and that there is sufficient friction in the braking surfaces to keep the elevator in its position also with large loads of the elevator car.
The functionability of brake pads is analyzed in publication WO 03/059713. In the method the dielectric constant of the brake pad material is measured. When the value of the dielectric constant of a new brake pad that is in good condition is known, the measured value can be compared to it. If the measured value is not within the desired tolerance range around the reference value, it is concluded that the operating capability of the brake pads has deteriorated and in that case a service operative can be called to the site immediately.
The purpose of publication EP 1127025 is to present a holding brake, in which the degree of wear of the brake is measured constantly and thus the need for servicing can be forecast. In the method the position of the brake shoe is measured with a sensor in the braking state and in the releasing state. Based on these the quality of the operation of the brakes and the condition of the brake shoe and the braking surfaces can be determined.
Publication EP 502282 presents an appliance for monitoring the condition of brakes, to which a traction sheave rotated by the motor is connected. Wear of the friction surface of the brake is monitored with wear sensors. The position of the brake pads in relation to the traction sheave is observed with position sensors. By using simulated fault signals the monitoring signals given by the sensors are inspected at certain intervals and also the operability of the monitoring equipment itself. When the system identifies abnormal wear of the friction surface, an abnormal gripping attitude of the brake pads or if the appliance does not react to the simulated fault signals, the motor is switched off.
A drawback of prior-art methods for inspecting the condition of brakes is that they expose the appliance to a direct safety risk. Furthermore prior-art measuring and maintenance arrangements incur large extra costs.